1. Field of the Invention
The present invention relates to a process for separating water from an aqueous solution of an organic substance or a gaseous mixture of water with an organic substance. Particularly, the present invention relates to a separatory membrane used in the above process. More particularly, it relates to a permeable membrane through which water or water vapor can permeate selectively.
2. Description of the Prior Art
With respect to the concentration and separation of an aqueous solution of an organic substance with a membrane, a reverse osmosis method has been practically used for the concentration of some aqueous solutions of an organic substance having a low concentration. However, the reverse osmosis method requires that a pressure higher than the osmotic pressure of the separated solution be applied to the solution to be separated, so that the method can not be applied to an aqueous solution having a high concentration, i.e., a high osmotic pressure. Thus, the concentration of the solution, to which the method can be applied, has an upper limit.
Meanwhile, pervaporation and vapor permeation, both of which are not affected by osmotic pressure, have been spotlighted as new separation methods. Pervaporation is a process which comprises feeding a liquid mixture to be separated to the primary side of a membrane and reducing the pressure of the secondary side (permeation side) of the membrane or passing a carrier gas through the secondary side to thereby make the substance to be separated permeate through the membrane in a gaseous state, while the vapor permeation is different from the above pervaporation in that the mixture to be fed to the primary side of a membrane is gaseous. The membrane-permeating substance can be recovered by cooling and condensing the permeating vapor.
Many studies with respect to the pervaporation have been reported. For example, U.S. Pat. Nos. 3,750,735 and 4,067,805 disclose separation of an organic substance from water with a polymer having an active anionic group, while U.S. Pat. Nos. 2,953,502 and 3,035,060 disclose separation of ethanol from water with a membrane made of cellulose acetate or polyvinyl alcohol. Further, Japanese Patent Laid-Open No. 109,204/1984 discloses a cellulose acetate membrane and a polyvinyl alcohol membrane, while Japanese Patent Laid-Open No. 55,305/1984 discloses a crosslinked polyethyleneimine membrane. However, these membranes all exhibit poor separation performances, i.e., a low permeation rate and a low separation coefficient, thus being deficient in practicality.
On the other hand, as a membrane excellent in separation performances, Japanese Patent Laid-Open No. 129,104/1985 discloses a membrane made of an anionic polysaccharide. However, a membrane made of a polysaccharide or its derivative essentially has unavoidable problems inherent in natural polymers, for example, depolymerization by acid or alkali or decomposition by fungi, so that the endurance and chemical resistance thereof do not meet expectations.
As described above, the separatory membranes for pervaporation or vapor permeation according to the prior art must have large areas owing to their low permeation rates. Further, in the concentration of a solution to a desired extent with the separatory membrane according to the prior art, the permeation solution of a high concentration must be circulated and re-treated owing to its low separation coefficient. The fulfilment of these requirement disadvantageously enhances the cost of equipment or operation.
The term "permeation rate" as used in this specification refers to the amount of a mixture permeating a membrane per unit area and per unit time and is shown by kg/m.sup.2 .multidot.hr, while the term "separation coefficient (.alpha.)" refers to the ratio of water to an organic substance contained in the permeation gas to that of the same substances present in the feed solution or vapor, i.e., .alpha.=(X/Y.sub.p /(X/Y).sub.f, wherein X and Y stand for proportions of water and an organic substance in the two-component system, respectively, and p and f stand for "permeation" and "feed", respectively.
An object of the present invention is to provide a separatory membrane used for separating water from an aqueous solution of an organic substance or a gaseous mixture of an organic substance with water by pervaporation or vapor permeation, wherein said membrane exhibits a sufficiently enhanced endurance, a high permeation rate and a high separation coefficient over a wide concentration range of an organic substance in a feed solution.
In order to make the water contained in an aqueous solution of an organic substance or in a gaseous mixture of an organic substance with water permeate a membrane selectively, it is preferred to introduce a functional group having a high water-coordinating power into the membrane. The water coordinated with a membrane is called bonded water, as contrasted with free water present in a bulk solution. It is presumed that the introduction of a dissociable group which excludes an organic substance and selectively coordinates water will increase the amount of the bonded water present in a membrane, thus remarkably enhancing the selective water-permeability of the membrane. On the basis of this presumption, it is described in the above-mentioned U.S. Pat. Nos. 3,750,735 and 4,067,805 that the separation coefficient of a nonionic polymer membrane can be enhanced by introducing an anionic group therinto. However, most polymers having numerous dissociable groups, such as polyacrylic acid, polymethacrylic acid, carboxymethylcellulose or their salts are soluble in water or highly swellable with water. Therefore, when the aqueous solution to be separated has a high concentration, a membrane made of such a polymer can be used in separation of water from the solution. However, when the solution to be separated has a low concentration, it is dissolved or significantly swollen to exhibit a remarkably lowered separating function. Although the resistance of such a membrane to an aqueous solution of an organic substance of a wide concentration range can be enhanced by converting the polymer of the membrane into a three-dimensional polymer by crosslinking, the crosslinked membrane generally tends to exhibit a lowered permeation rate.